1. Field of the Invention
This invention relates to solid electrolytic capacitors which have much improved frequency and temperature characteristics and a prolonged life.
2. Description of the Prior Art
A recent trend toward the digitalization of electronic apparatus requires capacitors which have a reduced impedance in a high frequency range. As is known in the art, electrolytic capacitors have been used in various electronic apparatus as bypass capacitors because of the small size and the large capacitance. These electrolytic capacitors are also required to have an improved high frequency impedance at low temperatures, stability at high temperatures and life stability over a long term owing to the great development of recent electronic apparatus. Several types of capacitors are now used for applications in high frequency ranges, including plastic film capacitors, mica capacitors, layer-built ceramic capacitors and the like. However, the film and mica capacitors are so large in size that it is difficult to have a large capacitance. The layer-built ceramic capacitors have a very poor temperature characteristic when fabricated to have a large capacitance, and are expensive. On the other hand, aluminum dry electrolytic capacitors or tantalum solid electrolytic capacitors are enabled to have a large capacitance using a very thin anodized dielectric film. However, the dielectric film is apt to damage, so that it is necessary to often repair the damage by providing an electrolytic layer between the anodized film and a cathode. For instance, with aluminum electrolytic capacitors, an anode and a cathode whose surface area is increased by etching are convolutely wound through separators to give a unit. This unit is dipped in a liquid electrolyte to obtain a capacitor element. This involves several drawbacks such as an increase of impedance at high frequency or low temperature caused by the ion conductivity of the electrolyte, and a decrease of electrostatic capacitance or an increase of dielectric loss with time as a result of the leakage of the electrolyte. These drawbacks place a limit on use as the industrial capacitor. In this sense, an aluminum or tantalum solid electrolyte capacitor may be a kind of small-size and large-capacitance capacitor which overcomes the drawbacks of the above-described aluminum liquid electrolytic capacitor, but this has several drawbacks. In the fabrication of the solid electrolytic capacitor, an anode is immersed in an aqueous solution of manganese nitrate, followed by thermal decomposition in a high temperature furnace at approximately 350.degree. C. to form a solid electrolyte layer consisting of manganese dioxide. The solid electrolyte layer exhibits much better frequency, temperature and life characteristics than liquid electrolytes. However, because the anodized film is inevitably damaged during several cycles of the thermal decomposition at high temperatures and manganese dioxide has a high specific resistance and behaves as a semiconductor with regard to electric conductivity, the impedance or dielectric loss in a high frequency or low temperature range is much higher than in the case of the film capacitor.
In order to avoid these drawbacks of the known capacitors, there has been proposed the use of organic solid semiconductors which have high conductivity and good anodizability. Especially, organic semiconductors consisting of tetracyanoquinodimethane (hereinafter abbreviated as TCNQ) complex salts have a number of advantages when used as the solid electrolyte for these purposes. For instance, the TCNQ complex salts may be applied to an anodized film by immersion in a solution of the salt in an organic solvent or in a melt of the salt by application of heat. This will mitigate the damage of the anodized film as will be caused by the thermal decomposition in the formation of manganese dioxide as described above. Moreover, the use of the TCNQ salt having electric conductivity similar to metals enables one to make capacitors of large capacitance having good high frequency characteristics.
The TCNQ salts suitable for these purposes are described, for example, in U.S. Pat. No. 3,872,358, Great Britain Pat. No. 2113916A and DE-OS No. 3214355. In these publications, there are used complex salts which comprise a cation of quinoline, isoquinoline or pyridine whose N position is substituted with an alkyl group and two TCNQ molecules. These salts have clear melting points when the alkyl group has 3 or more carbon atoms. Accordingly, they may be impregnated in a capacitor unit after melting, thereby forming a uniform solid electrolyte layer on the anodized film. The solid electrolyte-impregnated unit is used after covering with polymeric sealing materials by dipping, casting or potting in order to impart good resistances to humidity, heat and impact to the unit. The sealing materials are thermosetting resins including epoxy resins.
However, when the TCNQ-base solid electrolytes are used singly, several drawbacks are produced. For example, with a solid electrolytic capacitor obtained by impregnation with melt of a TCNQ salt, the composition of the TCNQ salt tends to be changed on melting, thus causing the inherent characteristics of the TCNQ salt to deteriorate.
The complex salt of TCNQ has a molar ratio of a cation and TCNQ of approximately 1:2. This ratio may vary by application of heat, so that the high conductivity of the TCNQ salt lowers. This is because one molecule of the two TCNQ molecules in the TCNQ salt is similar to neutral TCNQ in nature and is thus bonded only weakly in the semiconductor salt, so that when the salt is heated to a temperature of approximately 100.degree. C. or over, the molecule tends to split off from the salt. In order to prevent the splitting of the TCNQ molecules, it is necessary to rapidly heat and melt the salt and to quickly quench the melt after application. However, this manner of application is not sufficient for the prevention. The problems involved in the splitting of the TCNQ molecules is as follows: the conductivity of TCNQ salt lowers to about 1/10 the original conductivity; and the anodizability lowers with a lowering of storage stability. The resulting solid electrolytic capacitor has thus a vital disadvantage of increasing an impedance and leakage current.
Other problems of the known TCNQ salt electrolytic capacitors reside in generation of toxic gases as a result of the thermal decomposition of TCNQ salts. If TCNQ salts are incidentally exposed to high temperatures such as by fire or by generation of heat through passage of overcurrent, the nitrile groups (--CN) in TCNQ are separated, with the possibility of emitting toxic gases or vapors such as hydrogen cyanide, acetonitrile and the like.
In the above prior capacitors, a TCNQ salt is impregnated in an anodized film or surface after melting of the salt. More specifically, after melting of the salt, an anodized film is immediately immersed in the melt and the immersion is completed within a short time before the thermal decomposition of the TCNQ salt takes place. Thereafter, the immersed film is quickly cooled to form fine crystals of the salt on and in the film. However, the melt of the TCNQ salt has generally a high viscosity and thus the immersion within such a short time as mentioned above is insufficient for impregnation in the anodized film.